Tetrafunctional initiator

ABSTRACT

The melt of polyvinyl aromatic polymers comprising from 10 to 45 weight % of star branched polymer prepared using a combination of thermal and tetra functional peroxide initiation has an improved melt strength permitting better foam formation for extrusion foam blown with conventional blowing agents and inert gases including CO 2 .

This is a division, of application Ser. No. 09/836,611, filed Apr.16,2001, which is a division of Ser. No. 09/678,910 filed Oct. 4, 2000, nowU.S. Pat. No. 6,274,641, which is a division of Ser. No. 09/553,593filed Apr. 20, 2000, now U.S. Pat. No. 6,166,099.

FIELD OF THE INVENTION

The present invention relates to polymeric foams. More particularly thepresent invention relates to foams of vinyl aromatic polymers thatcomprise from 10 to 45 weight % of a star branched vinyl aromaticpolymer.

BACKGROUND OF THE INVENTION

In the manufacture of extrusion foam there are competing factors tobalance. One needs to consider the viscosity or melt flow rate of thepolymer as it impacts on the extruder output and the melt strength ofthe polymer, and particularly of the foaming mass as it leaves theextruder as this impacts on the bubble stability or the foam stability.If one makes a very low viscosity polymer it will flow through theextruder easily. However a low viscosity polymer tends to have a lowmelt strength and the resulting foam tends to have a lower stability.Accordingly, there is a tendency for foams of low viscosity to collapseupon extrusion or shortly after leaving the extruder.

It has been known for some time that the melt strength of a polymer maybe improved by lightly cross linking the polymer. The paper “SomeEffects of Crosslinking Upon the Foaming Behavior of Heat PlastifiedPolystyrene”, L. C. Rubens Journal of Cellular Plastics, April 1965,311-320 discloses that polystyrene, containing small amounts (about 0.03weight %) of divinyl benzene, may be foamed with CO₂ and the polymer hasgood foam stability and good foam volume. This technology is also thesubject matter of U.S. Pat. Nos. 2,848,427 and 2,848,428 issued Aug. 19,1958 to Louis C. Rubens assigned to The Dow Chemical Company. Thetechnology comprised forming a cross linked polystyrene polymer thenimpregnating it in solid state with CO₂ then releasing the pressure andletting the polymer expand. This technology was not strongly relevant toextrusion foam techniques.

The cross linking technology was further applied in U.S. Pat. No.3,960,784 issued Jun. 1, 1976 to Louis C. Rubens assigned to The DowChemical Company. This patent teaches concurrent impregnation of apolymer with a blowing agent and a cross linking agent. The polystyreneis prepared at temperatures from about 60° C. to 120° C., preferablyfrom about 70° C. to 100° C. (Column 3 lines 25-26). These temperatureranges are indicative of suspension polymerization and concurrent orpost polymerization impregnation with the blowing agent and crosslinking agent (see Example 3) although the polymer could be molded intothin sheets for the impregnation step. This reference does not teach anextrusion foam.

While divinyl benzene is useful in suspension polymerization it tends toproduce gels in bulk or solution polymerization. In a bulk or solutionpolymerization the use of tetra functional initiators significantlyreduces gels. Typically no or very low levels (e.g. less than 0.5 weight%, more generally less than 0.1 weight %) of gels (i.e. insolublepolymer in typical solvents).

With the Montreal protocol on reducing the use of CFC's and HCFC's andregulations regarding the permissible discharge of volatile organiccompounds (VOC's) there was increase pressure on polymer foam industryto move to other blowing agents such as CO₂ or N₂. Representative ofthis type of art is Monsanto's Australian Patent 529339 allowed Mar. 17,1983. The patent teaches the formation of a foam by extrudingpolystyrene and injecting CO₂ into the extruder. Interestingly there isno mention of cross linking agents or branched polystyrene in thepatent. U.S. Pat. No. 5,250,577 issued Oct. 5, 1993 to Gary C. Welsh issimilar as it pertains to extrusion foaming polystyrene in an extrusionprocess using CO₂ as the sole blowing agent. Again there is no referencein U.S. Pat. No. 5,250,577 to the use of cross linking agents.

At about this time U.S. Pat. No. 5,266,602 issued to Walter et al.assigned to BASF. The patent teaches foaming a branched polystyrene. Thefoaming agent is conventional (e.g. C₄₋₆ alkanes). The polymer isprepared in the presence of a peroxide initiator other than a benzoylcompound and a moderator (chain transfer agent) such as a mercaptan(e.g. t-dodecyl mercaptan) and a “branching agent”. The branching agentcontains a second unsaturation as a point for the polymer to branch.Suitable agents include divinyl benzene, butadiene and isoprene. Thesetypes of branching agents would not produce the star branched polymersreferred to herein. The actual polymerization process appears to be asuspension process. Additionally there is no reference in the disclosureto blowing the polystyrene with anything other than conventional alkaneblowing agents.

U.S. Pat. No. 5,576,094 issued Nov. 19, 1996 to Callens et al. assignedto BASF teaches extruding slab foamed polystyrene blown with CO₂ or amixture of CO₂ and C₁₋₆ alcohols or ethers of C₁₋₄ alkyl alkoxycompounds. The polystyrene is a branched polystyrene preferably havingat least 50%, more preferably 60% of the polymer being a star branchedstyrene butadiene block polymer. The polymer has a VICAT softeningtemperature not greater than 100° C. This teaches against the subjectmatter of the present invention. Additionally the polymer has a meltindex MVI 200/5 of at least 5 ml/10 minutes.

U.S. Pat. No. 5,830,924 issued Nov. 3, 1998 to Suh et al. assigned toThe Dow Chemical Company claims a process for extruding a closed cellfoam using CO₂ or a mixture of CO₂, conventional alkane blowing agentsand a polystyrene in which from 50 to 100 weight % of the polystyrene isstar branched (i.e. branched). This teaches away from the subject matterof the present invention which requires a different type of polymer andlower weight % of star branched vinyl aromatic polymer.

U.S. Pat. No. 5,760,149 issued Jun. 2, 1998 to Sanchez et al. disclosestetrafunctional (monoperoxycarbonate) compounds that are useful asinitiators for olefin monomers including styrene. The patent alsoteaches a process for polymerizing polystyrene. However, there is noteaching in the patent of foaming the resulting polymer using extrusiontechniques.

The present invention seeks to provide a novel process for extrusionfoaming of styrenic polymers in which the styrenic polymer comprisesless than 50 weight % of branched styrenic polymer.

SUMMARY OF THE INVENTION

The present invention provides a closed cell foam comprising from C₈₋₁₂vinyl aromatic polymer comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acid andacrylonitrile and methacrylonitrile; which polymer may be grafted ontoor occluded within from 0 to 12 weight % of one or more rubbery polymersselected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidvinyl aromatic polymer comprising 10 to 45 weight % of a star branchedpolymer and having a VICAT softening temperature not less than 100° C.

In a further embodiment the present invention provides a process forpreparing the above closed cell foam comprising injection into a moltenmass of C₈₋₁₂ vinyl aromatic polymer comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acid andacrylonitrile and methacrylonitrile; which polymers are grafted ontofrom 0 to 12 weight % of one or more rubbery polymers selected from thegroup consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidpolymer comprising 10 to 45 weight % of a star branched polymer andhaving a VICAT softening temperature not less than 100° C.; at atemperature from 140 to 235° C. and a pressure from 1500 to 3500 psifrom 2 to 15 weight % of one or more blowing agents selected from thegroup consisting of C₄₋₆ alkanes, CFCs, HCFCs, HFCs, CO₂ and N₂ andmaintaining said C₈₋₁₂ vinyl aromatic polymer in a molten state andthoroughly mixing said blowing agent with said polymer and extrudingsaid mixture of blowing agent and polymer.

The present invention also provides a process for polymerizing a vinylaromatic monomer comprising from 5 to 45 weight % of star branched vinylaromatic polymer, comprising feeding a mixture comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acid andacrylonitrile and methacrylonitrile; which polymer may be grafted ontoor occluded within from 0 to 12 weight % of one or more rubbery polymersselected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₆conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, andfrom 0.01 to 0.1 weight % of a tetrafunctional peroxide initiator of theformula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals; and R is a neopentyl group, in the absence of a cross linkingagent to a series of two or more continuous stirred tank reactors, toprovide a relatively low temperature initial reaction zone at atemperature from 100 to 130° C. and a relatively higher temperaturesubsequent reaction zone at a temperature from 130 to 160° C. andmaintaining a ratio of residence time in said relatively lowertemperature reaction zone to said relatively higher temperature reactionzone from 1:1 to 3:1 and recovering the resulting polymer.

BEST MODE

As used in this specification “star branched” polymer means havingmultiple, preferably at least 3, most preferably 4, branches eminatingfrom a common node.

The styrenic polymers of the present invention may be co- orhomopolymers of C₈₋₁₂ vinyl aromatic monomers. Some vinyl aromaticmonomers may be selected from the group consisting of styrene, alphamethyl styrene and para methyl styrene. Preferably the vinyl aromaticmonomer is styrene.

The styrenic polymer may be a copolymer comprising from 60 to 100 weight% of one or more C₈₋₁₂ vinyl aromatic monomers; and from 0 to 40 weight% of one or more monomers selected from the group consisting of C₁₋₄alkyl esters of acrylic or methacrylc acid and acrylonitrile andmethacrylonitrile. Suitable esters of acrylic and methacrylic acidinclude methyl acrylate, ethyl acyrlate, butyl acrylate, methylmethacrylate, ethyl methacrylate and butyl methacrylate.

The polymers of the present invention may be rubber modified. That isthe polymer may be grafted onto or occluded within from 0 to 12 weight %of one or more rubbery polymers selected from the group consisting of:

i) co- and homopolymers of C₄₋₅ conjugated diolefins; and

ii) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile.

The rubbery polymer may be selected from a number of types of polymers.The rubbery polymer may comprise from 40 to 60, preferably from 40 to 50weight % of one or more C₈₋₁₂ vinyl aromatic monomers which areunsubstituted or substituted by a C₁₋₄ alkyl radical and from 60 to 40,preferably from 60 to 50 weight % of one or more monomers selected fromthe group consisting of C₄₋₅ conjugated diolefins. Such polymers areknown as the styrene butadiene rubbers (SBR). The rubber may be preparedby a number of methods, preferably by emulsion polymerization. Thisprocess is well known to those skilled in the art and described forexample in Rubber Technology, Second Edition, edited by Maurice Morton,Robert E. Krieger Publishing Company Malabar, Florida, 1973, reprint1981—sponsored by the Rubber Division of the American Chemical Society.

The rubbery polymer may be a nitrile rubber comprising from 15 to 40weight % of one or more monomers selected from the group consisting ofacrylonitrile and methacrylonitrile, preferably acrylonitrile, and from85 to 60 weight % of one or more C₄₋₆ conjugated diolefins. The polymersmay be prepared by a number of methods, preferably by emulsionpolymerization. This process is well known to those skilled in the artand described for example in the aforementioned reference.

The rubber may be a co- or homopolymer of one or more C₄₋₆ conjugateddiolefins such as butadiene (1,3-butadiene) or isoprene, preferablybutadiene. The polybutadiene may have a molecular weight (Mw) from about260,000 to 300,000, preferably from about 270,000 to 280,000.Polybutadiene has a steric configuration. The polymer may have a cisconfiguration ranging from about 50% up to 99%. Some commerciallypolymers have a cis content of about 55% such as TAKTENE® 550 (trademarkof Bayer AG) or DIENE® 55 (trademark of Firestone). Some commerciallyavailable butadiene has a cis configuration from about 60 to 80% such asFirestone's DIENE® 70. Some high cis-butadiene rubbers may have a cisconfiguration of 95% or greater, preferably greater than 98% (TAKTENE®1202).

If present, preferably the rubber is present in an amount from about 3to 10 weight % based on the total composition fed to the reactor (i.e.monomers and rubber). Polybutadiene is a particularly useful rubber.

The process for making HIPS is well known to those skilled in the art.The rubber is “dissolved” in the styrene monomer (actually the rubber isinfinitely swollen with the monomer). This results in two co-continuousphases. The resulting “solution” is fed to a reactor and polymerizedtypically under shear. When the degree of polymerization is about equalto the weight % of rubber in the system it inverts (e.g. thestyrene/styrene polymer phase becomes continuous and the rubber phasebecomes discontinuous. After phase inversion the polymer is finished ina manner essentially similar to that for finishing polystyrene.

The polymer is prepared using conventional bulk, solution, or suspensionpolymerization techniques. However, there is added to the first reactor(i.e. the lower temperature reactor) from about 0.01 to 0.1 weight %(100 to 1000 ppm) of a tetrafunctional peroxide initiator of theformula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group. The reaction is conducted in theabsence of a cross linking agent. Preferably the tetrafunctionalperoxide is present in the feed to the first reactor (i.e. the lowertemperature reactor) in an amount from about 200 to 400 ppm (0.02 to0.04 weight %), most preferably from 250 to 350 ppm (0.025 to 0.035weight %).

Suitable tetrafunctional peroxide initiators include initiators selectedfrom the group consisting of tetrakis-(t-amylperoxycarbonyloxymethyl)methane, tetrakis-(t-butylperoxycarbonyloxymethyl) methane,1,2,3,4-tetrakis (t-amylperoxycarbonyloxy) butane and the tetrakis(t-C₄₋₆ alkyl monoperoxycarbonates). A particularly useful initiator isthe compound of the above formula wherein R is a nenopentyl group and R¹is a tertiary amyl or tertiary butyl radical.

Typically in a bulk or solution process the monomer mixture andoptionally rubber is polymerized in at least two continuous stirred tankreactors. The first reaction temperature is kept at a relatively lowtemperature from about 100 to 130° C., preferably from 120 to 130° C.and then at a relatively higher temperature from about 130 to 160° C.,preferably from about 135 to 145° C. In the polymerization process thereare competing initiation reactions. The initiation may be thermalwithout the use of the initiator or it may be initiated by the peroxycarbonate initiator. The residence time in each temperature zone iscontrolled so that the amount of polymerization initiated thermally(which results in a linear polymer) and by the peroxy carbonateinitiator (in which about half of the resulting polymer is branched) iscontrolled so that not more than 50 weight % of the resulting polymer isbranched. For example if the reaction is controlled so that the ratio ofresidence time at the lower temperature to time at higher temperature isfrom 1:1 to 3:1, preferably from 1.5:1 to 2.5:1, most preferably about2:1 (i.e. 1.8:1 to 2.2:1). The weight ratio of linear to star branchedpolymer is controlled to greater than 1:1 (e.g. greater than 50:50).Preferably, the vinyl aromatic polymer or styrenic polymer will comprisefrom about 10 to 45, preferably from about 15 to 40, most preferablyfrom about 15 to 30 weight % of a star branched polymer.

In a suspension process the monomers, optionally including dissolvedrubber, may be either first partially polymerized in a continuouslystirred tank system. The partially polymerized monomer mixture hasstabilizers or suspending agents added to it to help suspend it in theaqueous phase as an oil in water suspension. Typically the stabilizer orsuspending agent is added in an amount from 0.1 to 2.0 weight %,preferably from 0.5 to 1.0 weight %.

Useful stabilizers or suspending agents are well known to those skilledin the art. Useful stabilizers or suspending agents include polyvinylalcohol, gelatin, polyethylene glycol, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl pyrrolidone, polyacrylamides, saltsof poly (meth) acrylic acid, salts of phosphonic acids, salts ofphosphoric acid and salts of complexing agents such as ethylene diaminetetraacetic acid (EDTA).

Generally the salts are ammonium, alkali and alkaline earth metal saltsof the foregoing stabilizers or suspending agents. For exampletricalcium phosphate is a suitable suspending agent.

The tetra functional initiator may be added to the monomer mixture priorto polymerization in the bulk or mass reactor or just prior tosuspension batch polymerization in the suspension batch reactor. Thesuspension batch reactor is generally operated at lower temperaturesthan the bulk reactor. However, the suspension batch reaction isfinished at higher temperatures from about 120 to 150° C., typicallyfrom about 125 to 135° C.

The resulting polymer has a number of unique properties that make itsuitable for extrusion foaming and particularly suitable for extrusionfoaming using inorganic blowing agents such as CO₂ or N₂. The polymerhas a VICAT softening temperature (as measured by DIN 53460 isequivalent to ISO 306 is equivalent to ASTM D 1525-96) of greater than100° C., preferably from 105° C. to 115° C. The polymer has a mean meltstrength at 210° C. of not less than 12.5 cN.

The melt strength and the stretch ratio test are determined using aRosand® Capillary Rheometer. The mean melt strength is determined byextrusion of a melt at 210° C. of the polymer through a circular 2-mmdiameter flat die, length to diameter (L/D) of the die is 20:1. Thestrand is extruded at a constant shear rate of 20 sec⁻¹. The strand isattached to a haul off unit which increases in speed with time. Thestrand is attached to a digital balance scale to measure the force ofdraw on the polymer. As the speed of the haul off unit increases thedraw force increases. As a result the strand breaks. The draw forceimmediately prior to break is defined as the melt strength. The stretchratio is defined as the ratio of the velocity of draw to the extrusionvelocity at the die exit. The test is repeated at least three times todetermine an average value.

The polymer may have a melt flow at condition G (230° C./5 Kg load ofless than 5 g/10 minutes, preferably less than 3 g/10 minutes, mostpreferably of less than 2 g/10 minutes. Additionally, the polymer has aMz which exceeds typical high heat crystal polystyrene resins by atleast 40,000, preferably by greater than 60,000.

The polymer may be foamed using conventional extrusion foamingequipment. The extruder may be a back to back type or it may be amultizoned extruder having at least a first or primary zone to melt thepolymer and inject blowing agent and a second extruder or zone.

In the primary extruder or zone the polymer melt is maintained attemperatures from about 425° F. to 450° F. (about 218 to 232° C.). Oncethe polymer is melted, blowing agent is injected into the melt at theend of the primary extruder or zone. In the primary extruder or zonethere will be a high shear zone to promote thorough mixing of theblowing agent with the polymer melt. Such a zone may comprise a numberof pin mixers.

The polymer melt containing dissolved or dispersed blowing agent is thenfed from the primary extruder to the secondary extruder or passes from aprimary zone to a secondary zone within the extruder maintained at amelt temperature of 269° F. to 290° F. (about 132° C. to 143° C.). Inthe secondary extruder or zone the polymer melt and entrained blowingagent passes through the extruder barrel by the action of an auger screwhaving deep flights and exerting low shear upon the polymer melt. Thepolymer melt is cooled by means of cooling fluid, typically oil whichcirculates around the barrel of the extruder. Generally the melt iscooled to a temperature of from about 250° F. to about 290° F. (about121° C. to 143° C.).

The blowing agent may be selected from the group consisting of C₄₋₈alkanes, CFCs, HFCs, HCFCs, CO₂, N₂, air and mixtures thereof. Theblowing agent may be CO₂ per se or N₂ per se. The blowing agent maycomprise from 20 to 95 weight % of a blowing agent selected from thegroup consisting of one or more C₄₋₆ alkanes (as described below) andfrom 80 to 5 weight % of CFCs, HFCs and HCFC's (as described below).Suitable C₄₋₆ alkanes include butane, pentane and mixtures thereof.

The blowing agent may comprise from 30 to 95, preferably from 70 to 95,most preferably from 80 to 90 weight % of CO₂ and from 70 to 5,preferably from 30 to 5, most preferably from 20 to 10 weight % of oneor more compounds selected from the group consisting of C₁₋₂ halogenatedalkanes and C₄₋₆ alkanes. Suitable C₁₋₂ halogenated alkanes include thechloroflurocarbons (CFCs); hydrofluorocarbons (HFCs) and thehydrochlorofluorocarbons (HCFCs) such as trichlorofluoromethane(CFC-11); dichlorodifluoromethane (CFC-12); trichlorotrifluoroethane(CFC-113); dichlorotetrafluoroethane (CFC-114); dichlorofluoromethane(CFC-21); chlorodifluoromethane (HCFC-22); difluoromethane (HFC-32);2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124); pentafluoroethane(HFC-125); 1,1,1,2-tetrafluoroethane (HCFC-124);1,1-dichloro-1-fluoroethane (HCFC-141 b); 1-chloro-1,1-difluoroethane(HCFC-142b); trifluoroethane (HFC-143a); 1,1-difluoroethane (HFC-152a);tetrafluoroethane (HFC-134a); and dichloromethane. However, due toenvironmental concerns it is preferred to use alkanes such as C₄₋₆alkanes which have not been halogenated such as butane, pentane,isopentane and hexane. The blowing agent system may be used in amountsfrom 2 to 15, preferably from 2 to 10, most preferably from about 3 to 8weight % based on the weight of the polymer.

The pressure within the extruder should be sufficient to keep theblowing agent in the polymer melt. Typically, the pressures in the meltafter the blowing system has been injected will be from about 1500 to3500 psi, preferably from about 2000 to about 2500 for C₂. The C₂ andthe other blowing agent may be injected separately into the melt. Ifthis is done, preferably the alkane and/or halogenated alkane will beinjected upstream of the C₂ as these types of blowing agents have aplasticizing effect on the polymer melt which may help the C₂ go intothe melt. The alkane blowing agent and the C₂ may also be mixed prior toinjection into the extruder as is disclosed in U.S. Pat. No. 4,424,287issued Jan. 3, 1984 assigned to Mobil Oil Corporation.

To improve the cell size and/or distribution throughout the polymersmall amounts of a nucleating agent may be incorporated into the polymerblend or solution. These agents may be physical agents such as talc orthey may be agents which release small amounts of CO₂ such as citricacid and alkali or alkaline earth metal salts thereof and alkali oralkaline earth metal carbonates or bicarbonates. Such agents may be usedin amounts from about 500 to 5,000 ppm, typically from about 500 to2,500 ppm based on the polymer melt or blend.

The polymer melt or blend may also contain the conventional additivessuch as heat and light stabilizers (e.g. hindered phenols and phosphiteor phosphonite stabilizers) typically in amounts of less than about 2weight % based on the polymer blend or solution.

The foam is generally extruded at atmospheric pressure and as a resultof the pressure release on the melt, the melt foams. The foam is cooledto ambient temperature typically below about 25° C., which is below theglass transition temperature of the polymer and the foam is stabilized.One of the advantages of the present invention is that the foamedpolymer melt has better melt strength than the foamed polymer melts ofthe prior art and there is less foam collapse.

The foam may be extruded onto rollers as a relatively thick slabtypically from about 1 to 3 inches thick. The foam density may vary from2 to 15 lbs/ft³ (from about 0.03 to 0.24 g/cm³). The slab is cut intoappropriate lengths (8 feet) and is generally used in the constructionindustry. Thinner foams, typically from about {fraction (1/16)} to about¼ inches (62 to 250 mils) thick may be extruded as slabs or as thinwalled tubes which are expanded and oriented over an expanding tubularmandrel to produce a foam tube which is slit to produce sheet. Theserelatively thin sheets are aged, typically 3 or 4 days and then may bethermoformed into items such as coffee cups, meat trays or “clamshells”.

The present invention will now be illustrated by the followingnon-limiting examples in which, unless otherwise indicated parts meansparts by weight (grams) and percent means weight percent.

EXAMPLE 1 Polymer Preparation

Styrene monomer and 0.028 weight % of a tetra t-alkylperoxy carbonatesold by Ato Chemie under the trade mark JWEB50 were first fed into acontinuously stirred tank reactor maintained at 120° C. The residencetime in the first reactor was about 2.5 hours. The partially polymerizedmixture from the first reactor was then fed to a second continuouslystirred tank reactor maintained at 140° C. The residence time in thesecond reactor was about 1 hour. The resulting polymer was thendevolatilized in a falling strand devolatilizer and recovered andpelletized.

The reaction conditions were such that about 64% of the polymer wasthermally initiated and linear. About 36% of the polymer was initiatedby the peroxide and about half of the resulting polymer was starbranched. The polymer had an Mz from 40,000 to 75,000 greater thanconventional high heat crystal.

EXAMPLE 2

The procedure of Example 1 was repeated except that the amount ofinitiator was 0.045 weight %.

EXAMPLE 3

The procedure of Example 1 was repeated except that zinc stearate wasalso included in the polymer in an amount of about 0.1 weight %.

Physical Properties

The physical properties of the resins prepared in Examples 1, 2 and 3were compared to NOVA Chemicals' VEREX™ 1280 resin and 1230 polystyreneresin (both linear crystal polystyrene resins used in extrusion foamapplications), and a Dow resin sold for use in extrusion foamapplications. The results are set forth in Table 1.

EXAMPLE 4

The above samples together with the reference samples were extrusionfoamed using pentane as the blowing agent. The average cell diameter ofthe foam was measured. The results are set out in Table 2.

The foams extruded well and the cell data suggests that the foamstability good. The resulting foams have good toughness.

TABLE 1 Polystyrene Sample Identification Example 1 Example 2 Example 3Initiator: polyether tetrakis 280 ppm 450 ppm 280 ppm VEREX 1280 VEREX1230 Dow Resin Initiator Initiator Initiator + Zn (t-butylperoxycarbonate) Mw 351,000 345,000 342,000 306,000 309,000 310,000 Mn 132,000113,000 141,000 77,000 102,000 130,000 Mz 638,000 659,000 606,000535,000 551,000 550,000 Polydispersity (Mw/Mn) 2.66 3.05 2.42 3.97 3.032.38 Mean Melt Strength (cN) @ 190° C. 38.21 36.95 37.44 31.07 30.4234.7 Mean Stretch Ratio (%) @ 190° C. 91.8 81 79.9 84.3 99.4 91.8 MeanPeak Melt Strength @ 190° C. 45.25 45.73 43.7 38.74 34.75 39.73 MeanMelt Strength (cN) @ 210° C. 14.11 14 14.56 10.25 11.02 11.95 MeanStretch Ratio (%) @ 210° C. 279.6 230.3 236.5 428.6 399.4 326 Mean PeakMelt Strength @ 210° C. 17.43 17.02 17.73 12.22 12.87 14.43 Notched Izod(ft-lb/in) 0.36 0.32 0.34 0.33 0.32 0.22 Melt Flow Condition “G” (g/10min) 1.35 1.74 1.4 1.95 2.07 1.42 VICAT (° C.) 108.4 109 108.9 108.2108.6 109.9

TABLE 2 Polystyrene Sample Identification Example 1 Example 2 Example 3Initiator: polyether tetrakis 280 ppm 450 ppm 280 ppm Verex 1280 Verex1230 Dow Resin initiator initiator initiator + Zn (t-butylperoxycarbonate) Isopentane led to foam process (wt %) 5 5 5 5 5 5 TestResults on 2S Type Foamed Meat Trays Molded From Polystyrene SamplesMean Load at Max Load (lbs) 2.44 2.56 2.84 3.2 2.88 2.96 MeanDisplacement at Max Load (in.) 2.16 1.92 1.46 1.71 2.13 2.28 Mean Loadat 1.5″ Deflection (lbs) 2.2 2.44 2.82 3.16 2.67 2.62 Mean Slope(lbs/in) 3.02 3.86 3.58 3.74 3.61 3.47 Mean Part Weight (grams) 4.5214.595 4.59 4.96 4.758 4.93 Mean Sidewall Thickness (inches) 0.091 0.0870.105 0.102 0.103 0.098 Mean Foam Density (lbs/ft³) 3.2 3.56 2.92 3.073.194 Mean Orientation MD (%) 55.93 57.48 56.07 54.4 56.68 54.49 MeanOrientation TD (%) 57.15 57.2 56.55 54.73 56.47 52.29 Number of CellsAcross Sheet Thickness (TD) 21 20 38 22 31 23 Average Cell Diameter (mm)0.1101 0.1105 0.0702 0.1178 0.0844 0.1082 Number of Parts With SidewallCracks 0 1 0 3 0 2 Cell Structure coarse fine coarse coarse Cell Shapeslight slight spherical spherical spherical spherical elongationelongation Corner inversion Test on Trays - Failure 0 3 0 2 0 6 Rate/20

What is claimed is:
 1. A process for polymerizing a vinyl aromaticmonomer comprising from 5 to 45 weight % of star branched vinyl aromaticpolymer comprising feeding a mixture comprising: i) from 60 to 100weight % of one or more C₈₋₁₂ vinyl aromatic monomers; and ii) from 0 to40 weight % of one or more monomers selected from the group consistingof C₁₋₄ alkyl esters of acrylic or methacrylc acid and acrylonitrile andmethacrylonitrile; which polymer optionally grafted onto or occludedwithin from 0 to 12 weight % of one or more rubbery polymers selectedfrom the group consisting of: iii) co- and homopolymers of C₄₋₅conjugated diolefins; and iv) copolymers comprising from 60 to 85 weight% of one or more C₄₋₅ conjugated diolefins and from 15 to 40 weight % ofa monomer selected from the group consisting of acrylonitrile andmethacrylonitrile, and from 0.01 to 0.1 weight % of a tetrafunctionalperoxide initiator of the formula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group, in the absence of a cross linkingagent to a series of two or more continuous stirred tank reactors, toprovide a relatively low temperature initial reaction zone at atemperature from 100 to 130° C. and a relatively higher temperaturesubsequent reaction zone at a temperature from 130 to 160° C. andmaintaining a ratio of residence time in said relatively lowertemperature reaction zone to said relatively higher temperature reactionzone from 1:1 to 4:1 and recovering the resulting polymer.
 2. Theprocess according to claim 1, wherein the C₈₋₁₂ vinyl aromatic monomeris selected from the group consisting of styrene, alpha methyl styreneand para methyl styrene.
 3. The process according to claim 2, whereinthe C₈₋₁₂ vinyl aromatic polymer is a homopolymer.
 4. The processaccording to claim 3, wherein the C₈₋₁₂ vinyl aromatic polymer ispolystyrene.
 5. The process according to claim 4, wherein thetetrafunctional initiator is selected from the group consisting oftetrakis-(t-amylperoxycarbonyloxymethyl)methane andtetrakis-(t-butylperoxycarbonyloxymethyl)methane.
 6. The processaccording to claim 5, wherein the star branched vinyl aromatic polymeris present in an amount from 15 to 40 weight % of the vinyl aromaticpolymer.
 7. The process according to claim 1, wherein the rubberypolymer is present in an amount from 3 to 10 weight %.
 8. The processaccording to claim 7, wherein the C₈₋₁₂ vinyl aromatic monomer isselected from the group consisting of styrene, alpha methyl styrene andpara methyl styrene.
 9. The process according to claim 8, wherein theC₈₋₁₂ vinyl aromatic polymer is a homopolymer.
 10. The process accordingto claim 9, wherein the C₈₋₁₂ vinyl aromatic polymer is polystyrene. 11.The process according to claim 10, wherein the tetrafunctional initiatoris selected from the group consisting oftetrakis-(t-amylperoxycarbonyloxymethyl)methane, andtetrakis-(t-butylperoxycarbonyloxymethyl)methane.
 12. The processaccording to claim 11 wherein said rubbery polymer is polybutadiene. 13.The process according to claim 12, wherein the star branched vinylaromatic polymer is present in an amount from 15 to 40 weight % of thevinyl aromatic polymer.